1. © 2017 Pearson Education, Inc.

2. Living cells require energy from outside sources to do work
Life Is Work
Living cells require energy from outside sources to do work
The work of the cell includes assembling polymers, membrane transport, moving, and reproducing
Animals can obtain energy to do this work by feeding on other animals or photosynthetic organisms
© 2017 Pearson Education, Inc.

3. Figure 9.1 How does food, like the sand eels captured by this puffin, power the work of life?
© 2017 Pearson Education, Inc.

4. Light energy ECOSYSTEM Photosynthesis in chloroplasts Organic
Cellular respiration
in mitochondria
CO2 + H2O + O2
Organic
molecules
ATP
Heat
ATP powers
most cellular work
Figure 9.2 Energy flow and chemical recycling in ecosystems
© 2017 Pearson Education, Inc.

5. The breakdown of organic molecules is exergonic
Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels
The breakdown of organic molecules is exergonic
Fermentation is a partial degradation of sugars that occurs without O2
Aerobic respiration consumes organic molecules and O2 and yields ATP
Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O2
© 2017 Pearson Education, Inc.

6. C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy (ATP + heat)
Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration
Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy (ATP + heat)
© 2017 Pearson Education, Inc.

7. Redox Reactions: Oxidation and Reduction
Oxidation, a substance loses electrons
is oxidized
Reduction, a substance gains electrons
is reduced
(the amount of positive charge is reduced)
Reducing agent
The electron donor
Oxidizing agent
The electron receptor
© 2017 Pearson Education, Inc.

8. becomes oxidized (loses electron) becomes reduced (gains electron)
Figure 9.UN01 In-text figure, NaCl redox reaction, p. 166
© 2017 Pearson Education, Inc.

9. Oxidation of Organic Fuel Molecules During Cellular Respiration
Glucose is oxidized
O2 is reduced
becomes oxidized
becomes reduced
© 2017 Pearson Education, Inc.

10. Stepwise Energy Harvest via NAD+ and the Electron Transport Chain
In cellular respiration, glucose and other organic molecules are broken down in a series of steps
Electrons from organic compounds are usually first transferred to NAD+, a coenzyme
As an electron acceptor, NAD+ functions as an oxidizing agent during cellular respiration
Each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP
© 2017 Pearson Education, Inc.

11. (a) Uncontrolled reaction (b) Cellular respirationH2 +
½ O2
2 H +
½ O2
Controlled
release of
energy
2 H e–
Free energy, G
Free energy, G
Electron transport
chain
ATP
Explosive
release of
energy
ATP
ATP
2 e–
Figure 9.5 An introduction to electron transport chains
½ O2
2 H+
H2O
H2O
(a) Uncontrolled reaction
(b) Cellular respiration
© 2017 Pearson Education, Inc.

12. The Stages of Cellular Respiration: A Preview
Harvesting of energy from glucose has three stages
​Glycolysis (breaks down glucose into two molecules of pyruvate)
The citric acid cycle (completes the breakdown of glucose)
​Oxidative phosphorylation (accounts for most of the ATP synthesis)
© 2017 Pearson Education, Inc.

13. Electrons via NADH Electrons via NADH and FADH2 GLYCOLYSIS PYRUVATE
OXIDATION
OXIDATIVE
PHOSPHORYLATION
CITRIC
ACID
CYCLE
Glucose
Pyruvate
Acetyl CoA
(Electron transport
and chemiosmosis)
CYTOSOL
MITOCHONDRION
Figure 9.6_3 An overview of cellular respiration (step 3)
ATP
ATP
ATP
Substrate-level
Substrate-level
Oxidative
© 2017 Pearson Education, Inc.

14. Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration
A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation
For each molecule of glucose degraded to CO2 and water by respiration, the cell makes up to 32 molecules of ATP
© 2017 Pearson Education, Inc.

15. Enzyme ADP P ATP Substrate Product
Figure 9.7 Substrate-level phosphorylation
© 2017 Pearson Education, Inc.

16. Glycolysis occurs in the cytoplasm and has two major phases
Concept 9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
Glycolysis (“sugar splitting”) breaks down glucose into two molecules of pyruvate
Glycolysis occurs in the cytoplasm and has two major phases
Energy investment phase
Energy payoff phase
Glycolysis occurs whether or not O2 is present
© 2017 Pearson Education, Inc.

17. CITRIC ACID CYCLE OXIDATIVE PHOSPHORYL- ATION PYRUVATE OXIDATION
GLYCOLYSIS
Figure 9.UN06 In-text figure, mini-map, glycolysis, p. 170
ATP
© 2017 Pearson Education, Inc.

18. GLYCOLYSIS: Energy Investment Phase
Glucose
6-phosphate
Fructose
6-phosphate
ATP
Glucose
ADP
Figure 9.9aa_3 A closer look at glycolysis (part 1a, step 3)
Hexokinase
Phosphogluco-
isomerase 1 2
© 2017 Pearson Education, Inc.

19. GLYCOLYSIS: Energy Investment Phase
Glyceraldehyde
3-phosphate (G3P)
Fructose
6-phosphate
Fructose
1,6-bisphosphate
ATP
ADP
Isomerase 5
Phospho-
fructokinase
Aldolase
Dihydroxyacetone
phosphate (DHAP) 4
Figure 9.9ab_3 A closer look at glycolysis (part 1b, step 3) 3
© 2017 Pearson Education, Inc.

20. GLYCOLYSIS: Energy Payoff Phase2
ATP
Glyceraldehyde
3-phosphate (G3P) 2
NADH
2 NAD H+
2 ADP 2 2
Triose
phosphate
dehydrogenase
Phospho-
glycerokinase 2
P i
Isomerase 7 5
1,3-Bisphospho-
glycerate
3-Phospho-
glycerate 6
Aldolase
Dihydroxyacetone
phosphate (DHAP) 4
Figure 9.9ba_3 A closer look at glycolysis (part 2a, step 3)
© 2017 Pearson Education, Inc.

21. GLYCOLYSIS: Energy Payoff Phase2
ATP
2 H2O
2 ADP 2 2 2 2
Phospho-
glyceromutase
Enolase
Pyruvate
kinase 9
Figure 9.9bb_3 A closer look at glycolysis (part 2b, step 3) 8 10
3-Phospho-
glycerate
2-Phospho-
glycerate
Phosphoenol-
pyruvate (PEP)
Pyruvate
© 2017 Pearson Education, Inc.

22. Energy Investment Phase
Glucose 2
ATP used
2 ADP + 2 P
Energy Payoff Phase
4 ADP + 4 P 4
ATP formed
2 NAD e– + 4 H+ 2
NADH
+ 2 H+
Figure 9.8 The energy input and output of glycolysis
2 Pyruvate + 2 H2O
Net
Glucose
2 Pyruvate + 2 H2O
4 ATP formed – 2 ATP used
2 ATP
2 NAD e– + 4 H+
2 NADH + 2 H+
© 2017 Pearson Education, Inc.

23. Concept 9.3: After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules
Before the citric acid cycle can begin, pyruvate must be converted to acetyl coenzyme A (acetyl CoA), which links glycolysis to the citric acid cycle
This step is carried out by a multienzyme complex that catalyzes three reactions
Oxidation of pyruvate and release of CO2
Reduction of NAD+ to NADH
Combination of the remaining two-carbon fragment and coenzyme A to form acetyl CoA
© 2017 Pearson Education, Inc.

24. CITRIC ACID CYCLE OXIDATIVE PHOSPHORYL- ATION PYRUVATE OXIDATION
GLYCOLYSIS
Figure 9.UN07 In-text figure, mini-map, pyruvate oxidation, p. 171
© 2017 Pearson Education, Inc.

25. reduced oxidized -SH Pyruvate Dehydrogenase
Figure 9.10 Oxidation of pyruvate to acetyl CoA, the step before the citric acid cycle
Pyruvate Dehydrogenase
© 2017 Pearson Education, Inc.

26. NAD+ NADH Dehydrogenase Reduction of NAD+ 2[H] (from food)
Oxidation of NADH
Nicotinamide
(oxidized form)
Nicotinamide
(reduced form)
Figure 9.4 NAD+ as an electron shuttle
© 2017 Pearson Education, Inc.

27. The Citric Acid Cycle
The citric acid cycle, also called the Krebs cycle, completes the breakdown of pyruvate to CO2
The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn
© 2017 Pearson Education, Inc.

28. Figure 9.UN08 In-text figure, mini-map, citric acid cycle, p. 173
© 2017 Pearson Education, Inc.

29. Figure 9.12_1 A closer look at the citric acid cycle (step 1)
© 2017 Pearson Education, Inc.

30. Figure 9.12_2 A closer look at the citric acid cycle (step 2)
© 2017 Pearson Education, Inc.

31. Figure 9.12_3 A closer look at the citric acid cycle (step 3)
© 2017 Pearson Education, Inc.

32. Figure 9.12_4 A closer look at the citric acid cycle (step 4)
© 2017 Pearson Education, Inc.

33. Figure 9.12_5 A closer look at the citric acid cycle (step 5)
© 2017 Pearson Education, Inc.

34. Figure 9.12_6 A closer look at the citric acid cycle (step 6)
© 2017 Pearson Education, Inc.

35. Figure 9.12_7 A closer look at the citric acid cycle (step 7)
© 2017 Pearson Education, Inc.

36. Figure 9.12_8 A closer look at the citric acid cycle (step 8)
© 2017 Pearson Education, Inc.

37. Figure 9.12_8 A closer look at the citric acid cycle (step 8)

38. Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis
Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food
These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation
© 2017 Pearson Education, Inc.

39. The Pathway of Electron Transport
The electron transport chain is in the inner membrane (cristae) of the mitochondrion
Most of the chain’s components are proteins, which exist in multiprotein complexes
Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O
Electron carriers alternate between reduced and oxidized states as they accept and donate electrons
© 2017 Pearson Education, Inc.

40. CITRIC ACID CYCLE OXIDATIVE PHOSPHORYL- ATION PYRUVATE OXIDATION
GLYCOLYSIS
Figure 9.UN09 In-text figure, mini-map, oxidative phosphorylation, p. 174
ATP
© 2017 Pearson Education, Inc.

41. Electron transport chain
NADH
(least electronegative) 50 2 e–
NAD+
Complexes I-IV each consist of multiple proteins with electron
carriers.
FADH2 2 e–
FAD
Free energy (G) relative to O2 (kcal/mol) 40 I
FMN II
Fe•S
Fe•S Q
III
Cyt b 30
Fe•S
Cyt c1 IV
Cyt c
Cyt a
Electron transport
chain
Cyt a3 20
Figure 9.13 Free-energy change during electron transport 10 2 e–
2 H+ + ½ O2
(most electronegative)
H2O
© 2017 Pearson Education, Inc.

42. Electron transport chain 2 Chemiosmosis
INTERMEMBRANE SPACE H+
ATP
synthase
Protein
complex
of electron
carriers H+ H+ H+
Cyt c IV Q
III I II
2 H+ + ½ O2
H2O
FADH2
FAD
NADH
NAD+
Figure 9.15 Chemiosmosis couples the electron transport chain to ATP synthesis
ADP + P i
ATP
(carrying electrons
from food) H+ 1
Electron transport chain 2
Chemiosmosis
MITOCHONDRIAL MATRIX
Oxidative phosphorylation
© 2017 Pearson Education, Inc.

43. Chemiosmosis: The Energy-Coupling Mechanism
The energy released as electrons are passed down the electron transport chain is used to pump H+ from the mitochondrial matrix to the intermembrane space
H+ then moves down its concentration gradient back across the membrane, passing through the protein complex ATP synthase
H+ moves into binding sites on the rotor of ATP synthase, causing it to spin in a way that catalyzes phosphorylation of ADP to ATP
This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work
© 2017 Pearson Education, Inc.

44. The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work
© 2017 Pearson Education, Inc.

45. Stator H+ INTERMEMBRANE SPACE Rotor Internal rod Catalytic knob ADP +
Figure 9.14 ATP synthase, a molecular mill
ADP +
P i
ATP
MITOCHONDRIAL MATRIX
© 2017 Pearson Education, Inc.

46. An Accounting of ATP Production by Cellular Respiration
During cellular respiration, most energy flows in this sequence:
glucose → NADH → electron transport chain → proton-motive force → ATP
About 34% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 32 ATP
The rest of the energy is lost as heat
© 2017 Pearson Education, Inc.

47. Electron shuttles span membrane CYTOSOL MITOCHONDRION GLYCOLYSIS
2 NADH or
2 FADH2
6 NADH
MITOCHONDRION
GLYCOLYSIS
Glucose
2 Pyruvate
PYRUVATE
OXIDATION
2 Acetyl CoA
CITRIC
ACID
CYCLE
OXIDATIVE
PHOSPHORYLATION
(Electron transport
and chemiosmosis)
+ 2 ATP
Maximum per glucose:
About
30 or 32 ATP
+ about 26 or 28 ATP
Figure 9.16 ATP yield per molecule of glucose at each stage of cellular respiration
© 2017 Pearson Education, Inc.

48. There are three reasons why the number of ATP is not known exactly
Photophosphorylation and the redox reactions are not directly coupled; the ratio of NADH to ATP molecules is not a whole number
ATP yield varies depending on whether electrons are passed to NAD+ or FAD in the mitochondrial matrix
The proton-motive force is also used to drive other kinds of work
© 2017 Pearson Education, Inc.

49. Most cellular respiration requires O2 to produce ATP
Concept 9.5: Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen
Most cellular respiration requires O2 to produce ATP
Without O2, the electron transport chain will cease to operate
In that case, glycolysis couples with fermentation or anaerobic respiration to produce ATP
© 2017 Pearson Education, Inc.

50. Anaerobic respiration uses an electron transport chain with a final electron acceptor other than oxygen, for example, sulfate
Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP
© 2017 Pearson Education, Inc.

51. Lactic acid fermentation
Types of Fermentation
Fermentation consists of glycolysis plus reactions that regenerate NAD+
Which can be reused by glycolysis
Alcohol fermentation
Lactic acid fermentation
© 2017 Pearson Education, Inc.

52. In alcohol fermentation, pyruvate is converted to ethanol in two steps
The first step releases CO2 from pyruvate
The second step produces NAD+ and ethanol
Alcohol fermentation by yeast is used in brewing, winemaking, and baking
© 2017 Pearson Education, Inc.

53. (a) Alcohol fermentation 2 Acetaldehyde ATP
2 ADP + 2 P i 2
Glucose
GLYCOLYSIS
2 Pyruvate
2 NAD+
2 NADH
+ 2 H+
NAD+ REGENERATION
2 Ethanol
(a) Alcohol fermentation
2 Acetaldehyde
ATP
CO2
Figure 9.17 Fermentation
© 2017 Pearson Education, Inc.

54. In lactic acid fermentation, pyruvate is reduced by NADH, forming NAD+ and lactate as end products, with no release of CO2
Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt
Human muscle cells use lactic acid fermentation to generate ATP during strenuous exercise when O2 is scarce
© 2017 Pearson Education, Inc.

55. (b) Lactic acid fermentation
2 ADP + 2
P i 2
ATP
Glucose
GLYCOLYSIS 2
NAD+ 2
NADH
+ 2 H+
2 Pyruvate
Figure 9.17b Fermentation (part 2: lactic acid)
NAD+
REGENERATION
2 Lactate
(b) Lactic acid fermentation
© 2017 Pearson Education, Inc.

56. Animation: Fermentation Overview
© 2017 Pearson Education, Inc.

57. Comparing Fermentation with Anaerobic and Aerobic Respiration
All use glycolysis (net ATP = 2) to oxidize glucose and harvest chemical energy of food
In all three, NAD+ is the oxidizing agent that accepts electrons during glycolysis
The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration
Cellular respiration produces 32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule
© 2017 Pearson Education, Inc.

58. Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2
Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration
In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes
© 2017 Pearson Education, Inc.

59. Glucose Glycolysis CYTOSOL Pyruvate No O2 present: O2 present:
Fermentation
Aerobic cellular
respiration
MITOCHONDRION
Ethanol,
lactate, or
other products
Acetyl CoA
Figure 9.18 Pyruvate as a key juncture in catabolism
CITRIC
ACID
CYCLE
© 2017 Pearson Education, Inc.

60. The Evolutionary Significance of Glycolysis
Early prokaryotes likely used glycolysis to produce ATP before O2 accumulated in the atmosphere
Used in both cellular respiration and fermentation, it is the most widespread metabolic pathway on Earth
This pathway occurs in the cytosol so does not require the membrane-bound organelles of eukaryotic cells
© 2017 Pearson Education, Inc.

61. Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways
Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration
Carbohydrates
starch, glycogen, and several disaccharides
Proteins
Must be digested to amino acids and their amino groups must be removed
Fats are digested to glycerol (used to produce compounds needed for glycolysis) and fatty acids
Fatty acids
Broken down by beta oxidation
An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate
© 2017 Pearson Education, Inc.

62. Amino acids Fatty acids CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION
Proteins
Carbohydrates
Fats
Amino
acids
Sugars
Glycerol
Fatty
acids
GLYCOLYSIS
Glucose
Glyceraldehyde 3- P
NH3
Pyruvate
Figure 9.19_5 The catabolism of various molecules from food (step 5)
Acetyl CoA
CITRIC
ACID
CYCLE
OXIDATIVE
PHOSPHORYLATION
© 2017 Pearson Education, Inc.

63. Biosynthesis (Anabolic Pathways)
The body uses small molecules from food to build other their own molecules such as proteins
These small molecules may come directly from food, from glycolysis, or from the citric acid cycle
© 2017 Pearson Education, Inc.

64. Regulation of Cellular Respiration via Feedback Mechanisms
Feedback inhibition is the most common mechanism for metabolic control
If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down
Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway
© 2017 Pearson Education, Inc.

65. CITRIC ACID CYCLE Oxidative phosphorylation
Glucose
AMP
GLYCOLYSIS
Fructose 6-phosphate
Stimulates +
Phosphofructokinase – –
Fructose 1,6-bisphosphate
Inhibits
Inhibits
Pyruvate
ATP
Citrate
Acetyl CoA
Figure 9.20 The control of cellular respiration
CITRIC
ACID
CYCLE
Oxidative
phosphorylation
© 2017 Pearson Education, Inc.